Fix type.__subclasscheck__ (#5864)

Lib/test/test_typing.py

@@ -3296,8 +3296,6 @@ class ProtocolTests(BaseTestCase):
 
         self.assertNotIsInstance([], collections.abc.Mapping)
 
-    # TODO: RUSTPYTHON
-    @unittest.expectedFailure
     def test_issubclass_and_isinstance_on_Protocol_itself(self):
         class C:
             def x(self): pass

vm/src/builtins/type.rs

@@ -442,7 +442,7 @@ impl Py<PyType> {
     /// Determines if `subclass` is actually a subclass of `cls`, this doesn't call __subclasscheck__,
     /// so only use this if `cls` is known to have not overridden the base __subclasscheck__ magic
     /// method.
-    pub fn fast_issubclass(&self, cls: &impl Borrow<crate::PyObject>) -> bool {
+    pub fn fast_issubclass(&self, cls: &impl Borrow<PyObject>) -> bool {
         self.as_object().is(cls.borrow()) || self.mro.read().iter().any(|c| c.is(cls.borrow()))
     }
 
@@ -1216,8 +1216,10 @@ impl Py<PyType> {
     }
 
     #[pymethod]
-    fn __subclasscheck__(&self, subclass: PyTypeRef) -> bool {
-        subclass.fast_issubclass(self)
+    fn __subclasscheck__(&self, subclass: PyObjectRef, vm: &VirtualMachine) -> PyResult<bool> {
+        // Use real_is_subclass to avoid going through __subclasscheck__ recursion
+        // This matches CPython's type___subclasscheck___impl which calls _PyObject_RealIsSubclass
+        subclass.real_is_subclass(self.as_object(), vm)
     }
 
     #[pyclassmethod]

vm/src/protocol/object.rs

@@ -447,6 +447,12 @@ impl PyObject {
         }
     }
 
+    /// Real issubclass check without going through __subclasscheck__
+    /// This is equivalent to CPython's _PyObject_RealIsSubclass which just calls recursive_issubclass
+    pub fn real_is_subclass(&self, cls: &PyObject, vm: &VirtualMachine) -> PyResult<bool> {
+        self.recursive_issubclass(cls, vm)
+    }
+
     /// Determines if `self` is a subclass of `cls`, either directly, indirectly or virtually
     /// via the __subclasscheck__ magic method.
     /// PyObject_IsSubclass/object_issubclass